There are two main control methodologies for a switching power converter: voltage-mode control, and current-mode control. Voltage-mode control requires only one control loop such as performed in the comparison of a control signal to a ramp signal for pulse width modulating the power switch. An error amplifier compares the output voltage to a reference voltage to generate the control signal. In contrast, current-mode control requires two loops because one loop is necessary for feeding back the inductor current to form the control signal in addition to the loop feeding back the output voltage. Each control method has its own set of advantages and disadvantages. Hysteretic current-mode controllers in particular have advantageous response speed to load transients in the output voltage. In a hysteretic current-mode controller, the hysteresis of a comparator sets the ripple for the inductor current.
An example hysteretic current-mode controlled buck converter 100 is shown in FIG. 1. An operational transconductance amplifier (OTA) 105 generates an error current (Ierr) responsive to a difference between an output voltage Vout and a reference voltage Vref multiplied by its transconductance gain gm. The error current thus equals gm*(Vout−Vref). Should the output voltage be less than the reference voltage, the error current will thus discharge an inverting input to a hysteresis comparator 110 below its grounded non-inverting input such that the output of hysteresis comparator 110 is driven high. This high output of hysteresis comparator 110 switches on the high-side switch (S0) for buck converter 100 and also switches on switch S01 that couples between the inverting input to hysteresis comparator 110 and ground. With the high-side switch S1 on, an input voltage AVDD drives a magnetizing current Iind into an inductor L. A low-side switch S1 controlled by another hysteresis comparator 125 is off while the magnetizing current flows through high-side switch S0.
A current mirror 115 mirrors the magnetizing current Iind into a mirrored current Iind/N that a fraction 1/Nth of the magnetizing current. A hysteresis current Ih/N flows through switch S01. As the magnetizing current increases to be greater than a sum of the error current and the hysteresis current, the inverting input of hysteresis comparator is charged sufficiently above ground such that the output of hysteresis comparator 110 is pulled low, which switches off the high-side switch S1 and switch S01. An inverting input to hysteresis comparator 125 that is connected to the input node for inductor L1 is then charged low due to the stored magnetic energy in inductor L1 such that hysteresis comparator 125 switches on the low-side switch S1. The inductor current flowing through low-side switch S1 is then mirrored by another current mirror 120 into the current Iind/N. The resulting discharge by inductor L1 charges an output capacitor C with the output voltage Vout so that a load (represented by a resistor RLoad) may be powered accordingly.
Although the resulting hysteretic current-mode control for buck converter 100 advantageously responds to load transients, note that the switching frequency is indeterminate. In particular, the switching frequency will vary depending upon the input voltage and the load. It is thus known to modify hysteretic current-mode switching converters to achieve a fixed switching frequency but such modifications introduce control and implementation complexities.
Accordingly, there is a need in the art for improved hysteretic-controlled switching power converters having a fixed switching frequency.